136 LIFE BY THE SEASHORE. 



specimens. If you have a fair collection of these, you will 

 probably find among them some which present much the same 

 appearance as the living specimens, spines, mouth-membrane, 

 and teeth all being present as usual. In not a few cases, how- 

 ever, you will notice that the soft membrane shows signs of 

 decay either it cracks in dying, or it is attacked by sand- 

 hoppers or some of the shore insects. The result is to set 

 free the bulky and heavy lantern. This may then simply 

 fall out of the empty test, and be found lying intact on the 

 sand, or more probably its ligaments speedily decay and one 

 finds merely the scattered ossicles and teeth among the 

 wreckage. By the decay of the membrane the cavity of 

 the urchin is fully exposed, and the soft parts are speedily 



FIG. 44. Portions of Aristotle's lantern from a sea-urchin, 

 a, external view of the lantern, showing two of the five main 

 pieces (alveoli) of which it is composed ; b, internal view of 

 single piece ; c, side view ; t, in each figure, one of the five 

 chisel-edged teeth, which run through the alveoli and are 

 carried by them. 



eaten up, or dried up by the sun. The test then becomes 

 very light, is rolled over and over by the waves, so that the 

 spines are removed, and there is left the familiar empty 

 shell with a gaping orifice beneath, and a surface covered by 

 white knobs which show the places where the spines were 

 formerly attached. In other cases the disintegration of the 

 membrane is only partial, and the lantern merely falls into 

 the cavity of the urchin. Specimens of this kind often 

 occur with the lantern loose inside, and rattling at every 

 movement. As the lantern is heavy, the result in this case 

 is often to break the test in pieces, when the separated 

 waterworn pieces appear on the shore as what children call 

 "sailor's cheese." 



After this digression we may return to our living urchin. 



